Children at private schools are at greatest risk from the measles outbreak and could pose a health threat to the rest of the population, the north-west regional director of public health has warned.

Professor John Ashton said Britain's 600,000 privately-educated children were at much greater risk of infection than those in state schools, due to the many middle-class children whose parents did not vaccinate them because of the MMR vaccine scare in the 1990s and pupils from overseas with incomplete or unknown records.

Ashton, who represents the UK's public health doctors, added that private schools could become "reservoirs of disease".

He said the risk was similar to that from groups such as gypsies and travellers, who have acted as "reservoirs" for the infection.

Ashton, who will soon become president of the Faculty of Public Health, said middle-class parents who send their children to private schools were more likely to have avoided the MMR vaccine.

"Layered on top of that you have got a lot of children from abroad, especially from the far east, from countries such as Hong Kong and China, and there are few checks being done to establish their immunisation records," he told the Daily Telegraph.

He said private schools were a "law unto themselves", and warned both pupils and the wider population were being put at risk from infectious diseases because the schools do not have proper policies to protect children and are bad at keeping adequate medical records of pupils from abroad.

"The danger is that you have a population that can potentially become a reservoir of infection. Normally when you are talking about subsections of the population that are at particular risk of disease outbreaks, such as measles, you are talking about groups like gypsies and travellers. But actually children in private schools, and in particular boarding schools, are one of the categories most at risk."

He urged independent schools to check immunisation records of overseas pupils as a matter of urgency, and advised them to "engage with" families who refused to vaccinate their children.

Ashton's warning comes as health boards act to prevent a repeat of the outbreak of measles in the Swansea area where 942 cases have been reported. Vital vaccination sessions will also continue in a number of Swansea, Neath Port Talbot and north Powys schools next week.

A parallel programme is now being rolled out across three counties to the west of Swansea, while in England, a £20m vaccination programme has been announced to offer first time and top-up vaccinations.